A self-leveling fluid screed is formed from a cement composition that is based on a binder, a granulate, which is generally sand, at least one fluidizing adjuvant (also called superplasticizer or dispersant) and water. The fluidizing adjuvant makes it possible to transform this cement composition into a paste whose consistency after mixing is close to that of water. This explains the self-leveling and self-smoothing nature of the composition in the fresh state.
Such screeds are highly prized in the construction and building field for their great ease of use, their ability to correct surface evenness defects of supports and to coat, for example, ducts of heating floors. They make it possible to obtain, without outside intervention, the perfect spreading of the mixture that is therefore done by gravity owing to the composition of the poured mixture.
The implementation of these screeds can be done on sites according to two methods:                The first method consists in manufacturing a “premix” in a mixing station, i.e., to implement the dry mixing of all of the components of the cement composition: binders and adjuvants, with granulates, so as to be able to use a powdered mixture of ready-to-use mortar. It is delivered to the site in movable silos or in bags. This mortar is designed to be mixed on site mechanically with water and applied by pumping;        The second method consists in manufacturing the fluid mortar in a concrete plant and in delivering to the site via a mixing truck. The mortar is then generally pumped on the site of use by means of a pump.        
The different types of screeds are distinguished primarily by the nature of their binder.
The binders that are currently most used are natural or synthetic anhydrite (anhydrous calcium sulfate) that leads to the formation of gypsum after hydration, with the aluminous cement to which may or may not be added calcium sulfate that leads to the rapid formation of ettringite or Portland cement.
The so-called “anhydrite” screeds have the major drawback of being, after drying, very water-sensitive. Any rewetting of the screed by sweating from the support slab can give rise to serious disorders in the behavior of the ground covering. In addition, in drying, “anhydrite” screeds have protrusions of gypsum that crystallizes on the surface as well as protrusions of laitance (sweating that is manifested by an upwelling of water accompanied by fine elements of the formulation, such as limestone, gypsum). The surface formation of a skin that is not very resistant or powdered laitance restricts—before any placement of coating on said screed—operations of sanding, brushing or planing. Finally, the drying time of an anhydrite screed is generally slow. If the latter can be accelerated during the initial heating of a heating floor, in the absence of an internal heating system, the necessary drying time before coverage is approximately one month.
The use of aluminous cement, to which is generally added calcium sulfate, leads to the formation of ettringite after hydration. The resulting hardened screed is generally insensitive to water but in this case, the usable shelf life of the cement composition is rather short (less than 1 hour 30 minutes), making its use inadvisable in the case of transport by a cement mixer to the site from a concrete plant. The aluminous cement is then primarily used in the form of ready-to use “premix” and packaged in bags. It is mixed on site just before producing the screed.
The use of Portland cement by itself is generally preferable for producing a self-leveling fluid screed because it makes it possible to obtain a usable shelf life that is adequate for transporting and using the cement composition on site. In contrast, without the addition of specific adjuvants such as those described in, for example, the patent EP-B1-1,197,480, the hydration of Portland cement inevitably leads to the shrinkage phenomenon that gives rise to potential cracks, lifting of the edges of the screed, or warping and the subsequent formation of macrocracking under the action of its own weight. This pathology, also known as “curling,” is linked to the shrinkage differential between the bottom and the surface of the screed, a differential caused by a moisture gradient that develops during drying.
A first object of the invention is therefore to propose a cement binder for a self-leveling fluid screed that makes it possible to obtain a cement composition that has a usable shelf life of more than approximately 1 hour 30 minutes, in particular for enabling its preparation in a concrete plant, exhibiting a rapid hardening, reflected by a mechanical resistance to compression at 24 hours that is at least equal to approximately 1 MPa so as to enable the continuation of work on the site the next day by making possible at least pedestrian traffic.
To combat the dimensional variations during hydration and drying phases, so-called expansive cements have been developed. These cements, according to the ACI (American Concrete Institute) Committee 223—Standard Practice for the Use of Shrinkage-Compensating Concrete (ACI-223-98), are cements that, when they are mixed with water, produce a cement paste that, after setting, under moist curing conditions, tends to increase in volume. The expansion that is produced is capable of counterbalancing all or part of the shrinkage observed during exposure of the cement paste to an environment with reduced hygrometry and therefore obtaining cement compositions that have reduced dimensional variations. The expansive cement is thus described as a cement with shrinkage that is compensated to the extent that the residual expansion in the material remains limited and is even zero.
Furthermore, cement compositions with compensated shrinkage that are designed in such a way as to increase volume after setting and during the hardening phase at a young age are known. This expansion, when it is limited or restricted (by frames or simply a support or framework in the case of a slab), causes a compressive stress in the material. During subsequent drying, the shrinkage, instead of causing a traction stress that would lead to cracking, “releases” expansion deformations caused by the initial expansion.
Quicklime or magnesia is known as an expansive agent that makes it possible to compensate for shrinkage. As expansive binders, there are also expansive cements that lead to the controlled formation of etrringite in the first days that follow the installation of the concrete that make it possible to obtain a shrinkage-compensating effect. The three types of expansive cement that are recognized by the Standards ACI-223R6-93 and ASTM C 845-90 are as follows:                Type K: Contains C4A3$, C$, and non-combined lime (quicklime),        Type M: Contains calcium aluminates in CA and C12A7 form,        Type S: Contains a larger quantity of C3A than a Portland cement.        
In a K-type expansive cement—the most frequently encountered one, in particular in the United States—the formation of ettringite from C4A3$ is represented by the following equation:C4A3$+8C$H2+6CH+74H→3C6A$3H32 
The expansion of the cement paste that results from the formation of ettringite begins as soon as the water has been added, but the prevented expansion alone is beneficial, which is not the case as long as the concrete or mortar is in the plastic state. Furthermore, delayed expansion in a concrete can be dramatic when, for example, the latter undergoes an external attack by sulfates. It is therefore important that the formation of ettringite ceases after several days. During the use of this type of concrete, it is necessary to ensure curing with water, after pouring, a necessary condition for drawing full benefit from the use of such a cement. This limits the use of such cements to cement compositions that have a certain consistency. Consequently, until now, the case of self-leveling fluid cement compositions did not fall within the field of application of the cement compositions with compensated shrinkage.
Furthermore, the handling, or usability, of a cement composition that is manufactured with a K-type expansive cement is not as good, and the settling loss is greater than that observed with a traditional Portland cement. Taking into account the presence of quicklime in the K-type expansive cement, the kinetics of formation of the ettringite is quick and from the first moments leads to immobilizing a portion of the mixing water. This is reflected by a significant loss of usability over time.
A second object of this invention is to propose a binder for a self-leveling fluid screed with compensated shrinkage whose usable shelf life of the cement composition, implemented from this binder, can be controlled over an adequate period to make possible the use of said composition. Typically, this time is to easily reach 3 hours in the case of a delivery of the cement composition by mixing truck from the concrete plant (manufacturing site) to the work site (site of pouring the self-leveling fluid cement composition).
A third object of this invention is also to propose a cement composition that does not undergo a loss of fluidity over time: i.e., having a self-leveling nature from the time of its manufacturing until the time of its use at the site for making possible the casting of the screed.